When two terminals are communicating with each other, the selection of the modulation scheme used for the communication channels can significantly affect the throughput of that channel and also the ability of the channel to reject noise on the link, whether that be wireless or otherwise. In particular, it may be advantageous to use a modulation scheme which has a high throughput rate but with little error correction when the quality of the channel is good and, in contrast, a modulation scheme which has good resistance to noise albeit with lower throughput where the channel suffers noise or interference. The difficulty in selecting the appropriate modulation scheme comes about where the environment is variable such that in one instance, the noise may be very low but in another instance very high. Under these circumstances, the most reliable and noise resistant modulation technique must be used. However, this means that in those instances where the noise is low, the throughput is unnecessarily low.
Therefore, it is desirable to provide the ability to vary the modulation technique according to the current characteristics of a channel. In this way, if the channel is determined to be relatively free of the noise and interference then a modulation scheme with a high throughput can be used. Subsequently, if the noise or interference on the channel increases such that the current modulation scheme is unable to sustain reliable data transfer then the modulation scheme can be adapted to a more noise resistant modulation scheme.
However, this approach relies upon the respective transmit and receive links, referred to herein as the uplink and the downlink, between two terminals being subjected to similar interference or other factors such as noise. If there is a significant difference in the interference suffered by the uplink and downlink, considerable degradation of the throughput can occur. For example, in FIG. 1, two terminals A and B have a communication link formed between them. A source of interference I will affect the signals received by A and B. If, as shown in FIG. 1, the interfering source I is located centrally between the two terminals then the interference will have a similar effect on the signal received at both terminals. However, if the interference source moves (see FIG. 2) such that the interference received by one of the terminals, in this case B, is greater than that received at the other terminal then the interfering signal will affect the uplink more than the downlink. Consequently, unless the modulation scheme is very robust, the signal received by terminal B will include more errors. This may cause terminal B to determine that the signal path is significantly degraded and so switch to a more robust modulation scheme. However, the downlink, i.e. to terminal A, is still perfectly capable of supporting the previous modulation scheme. In fact, because the interferer has moved away from terminal A, the interfering signal has less of an effect on the signal received by A and so it may even be possible to switch to a different modulation scheme having a higher throughput.
The main disadvantage of the approach currently used, is that when applying adaptive modulation over a link conformed by up and down links, the response of the system can be severely deteriorated when not considering the differences that can occur between the links.